Role of defects on the electronic and magnetic properties of CrAs, CrSe and CrSb zinc-blende compounds

We present an extended study of single impurity atoms and atomic swaps in half-metallic CrAs, CrSb and CrSe zinc-blende compounds. Although the perfect alloys present a rather large gap in the minority-spin band, all defects under study, with the exception of void impurities at Cr and sp sites and Cr impurities at sp sites (as long as no swap occurs), induce new states within the gap. The Fermi level can be pinned within these new minority states depending on the lattice constant used for the calculations and the electronegativity of the sp atoms. Although these impurity states are localized in space around the impurity atoms and very fast we regain the bulk behavior, their interaction can lead to wide bands within the gap and thus loss of the half-metallic character.     

First-principles study of the thermal properties of half-metallic ferromagnetic systems

The origin of the half-metallicity is different in diluted magnetic semiconductors and Heusler alloys. I briefly review our earlier work on (GaMn)As and (GaMn)N focusing on the relation between the half-metallicity and the strength of the interatomic exchange interactions. This relation is governed by the properties of the valence-band holes. In Heusler alloys the factors determining the thermal behavior are distinct. Here the relation between half-metallicity and the longitudinal fluctuations of atomic moments is considered. The temperature dependence of the Ni magnetization in NiMnSb is studied.     

Defects-driven appearance of half-metallic ferrimagnetism in Co-Mn-based Heusler alloys

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co₂MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co_1−xMn_x]₂MnSi is of particular interest since Mn₃Si is known to crystallize in the Heusler L2₁ lattice structure of Co₂MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.     

Vacancy‐induced minority‐spin states in half‐metallic Heusler alloys

Ferromagnetic full Heusler alloys containing Co are amongst the most studied half‐metallic systems. Several studies recently have been concentrated on the effect of defects and impurities. We focus in this Letter on the case of vacancies in these alloys. We show that the occurrence of vacancies at the sites occupied by Co atoms can destroy half‐metallicity and alters the Slater–Pauling rule. Such defects are likely to occur since they result to the C1_b lattice structure of the semi‐Heusler alloys. Contrary, the appearance of vacancies at the other sites keeps the half‐metallic character of the parent alloys. Thus for realistic devices it is important to prevent the appearance of vacancies during the growth of thin films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Defect‐induced ferrimagnetism in the half‐metallic Co2CrAl and Co2CrSi compounds

Co₂CrAl and Co₂CrSi are amongst the most studied Heusler alloys due to their half‐metallic character. These compounds are also well‐known to present ferromagnetism with high Curie temperatures. We show using first‐principles calculations that the creation of Cr antisites (Cr atoms at the Co sites) induces ferrimagnetism in these compounds without destroying the half‐metallic character of these alloys. The reduction of the total spin moment causes lower external fields and thus smaller energy losses in realistic magnetoelectronic, also known as spintronic, devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic hyperfine fields for Sn atoms in rare-earth intermetallides: huge deviation from proportionality between Bhf and Sz

The magnetic hyperfine fields for ¹¹⁹Sn impurity atoms, localized in Ga sites of ferromagnetic intermetallic compounds RGa (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), were measured by the Mössbauer spectroscopy technique. At T=5 K, the hyperfine field value (B_hf) varies from 3.3 T in TmGa to 28.0 T in GdGa. Huge deviation from the proportionality between B_hf and the projection of the R³⁺ ion spin (S_z=(g−1)J) was found. As the atomic number of the R element increases, the B_hf/S_z ratio drastically decreases from 12.6 T for PrGa to 3.3 T for TmGa. This unexpected result can be explained by the strong dependency of B_hf value on the relationship between the Sn-R atomic separation (R_nn) and the radius of the magnetic 4f shell (R_4f). In the framework of this concept, the available experimental data for Sn atom in the rare-earth compounds with non-magnetic sp elements were considered. The data may be described by the universal dependency on the single parameter, λ=R_nn/R_4f.     

Electronic and magnetothermal properties of ferromagnetic clusters

The electronic structures and the magnetothermal properties of nickel clusters have been investigated. Their effective magnetic moments and specific heat capacities have been calculated assuming that the clusters undergo superparamagnetic relaxation. The average magnetic moments are computed adopting Friedel's model of ferromagnetic clusters. The surface effect and the cluster size effect on the thermodynamic properties of these clusters have been analysed based on the mean field theory approximation. The specific heat capacity of Ni clusters for N=300, where N is the number of atoms in the cluster, shows the peak value at T=550 K and exhibits a steady increase with N. The effective potentials and energy eigen values of the clusters as a function of the number of atoms and radius of the cluster have also been calculated self-consistently using the local density approximation (LDA) of the density functional theory (DFT); this has been performed within the framework of the spherical jellium background model (SJBM). The results of this study have been compared with the Stern-Gerlach experimental data and other theoretical results already reported in literature     

Huge positive hyperfine fields for Sn impurity atoms on R sites of R–T intermetallic compounds (R=rare-earth,T=Fe,Co)

The magnetic hyperfine field B_hf of the ¹¹⁹Sn impurity atom on the R site of the RFe₂ (R=Sm, Tb, Tm), TbCo₂, RCo₅ (R=Dy, Ho, Er), GdCo₃ and Gd₂Co₇ intermetallic compounds has been investigated by Mössbauer spectroscopy technique. At 5 K, very large hyperfine fields equal to 46–56 T were observed. The _Bhf$B_{\mathrm{hf}}$ values are several times larger than commonly observed for Sn in 3d-based magnetic hosts. The hyperfine fields are positive (that is parallel to the 3d magnetic moments direction). The results can be interpreted qualitatively in terms of the theory proposed for the impurity atoms in homogeneous ferromagnetic hosts [J. Kanamori, H. Katayama-Yoshida, K. Terakura, Hyperfine Interact. 8 (1981) 573; J. Kanamori, H. Katayama-Yoshida, K. Terakura, Hyperfine Interact. 9 (1981) 363; M. Akai, H. Akai, J. Kanamori, J. Phys. Soc. Jpn. 54 (1985) 4246; S. Blügel, H. Akai, R. Zeller, P.H. Dederichs, Phys. Rev. B 35 (1987) 3271], when it is considered that the splitting between bonding and antibonding hybrid states is strongly dependent on the interatomic distance. As the distance between the probe atom and neighboring magnetic atoms increases, the population of the antibonding states grows and, as a consequence, the corresponding positive contribution to the _Bhf$B_{\mathrm{hf}}$ increases sharply. For Sn atom the positive contribution to the _Bhf$B_{\mathrm{hf}}$ dominates when the interatomic distance exceeds 0.28–0.29 nm.     

Transformation of Co nanodisks to Co caterpillars

Co nanodisks and Co caterpillars were synthesized by novel wet chemistry method simply by controlling the concentration of cobalt ion. It was proven that both nanodisks and caterpillars had the same crystalline phase.     

First-principles investigations of Cr doping effects on electronic structure and magnetic properties in Sr2FeReO6

The electronic structures and magnetic properties of double perovskites Sr₂Fe_1−xCr_xReO₆ (x=0.0, 0.25, 0.5, 0.75, 1.0) have been studied within the local spin density approximation (LSDA) and LSDA+U schemes. The calculated results reveal that with increasing Cr content the cell volume shrinks 2.61%; the Fe/Cr site magnetic moment decreases while the Re-site moment increases. The total spin magnetic moment linearly decreases with the Cr doping from 3.00μB for x=0.00 down to 1.00μB for x=1.00 per formula unit. The magnetic coupling constants increase with increasing x. The electronic structure calculations indicate that the electronic concentration in the Re spin-down subband slightly increases resulting from the increase of bonding-antibonding interaction between the localised and the delocalised states in spin-down band; the coupling of O-2p_↑ and transition-metal-3d_↑ is substantially enhanced with the Cr doping. We discuss the origin of the anomalously high TC of Cr-doped Sr₂FeReO₆ compounds in terms of band hybridization effects.     

Crystal structure and the magnetic properties of CeTiGe3

The crystal structure of the RTiGe₃ compounds (R=La, Ce and Pr) has been studied by X-ray powder diffraction methods; Rietveld refinement has been carried out on the La homologue. These compounds crystallise in the BaNiO₃ prototype structure, hP10-P6₃/mmc, also called the hexagonal perovskite (a=6.300(1), c=5.915(1) Å for LaTiGe₃). This seems to be the first example in which an intermetallic phase adopts such a structure type which can be considered as derived from the Ni₃Sn type (anti) by a distortion of the lattice and the occupation of the quasi-octahedral 2a site at the origin of the cell (0, 0, 0) by Ti. The composition, also confirmed by microprobe analyses, was found to be strictly 1:1:3 indicating that these are line compounds, forming very likely by a peritectoid reaction. The existence of homologous compounds has been established for the lighter rare earths La, Ce and Pr. Heat capacity and magnetisation data show that CeTiGe₃ orders ferromagnetically with a Curie temperature of nearly 14 K. On the other hand a−ln T variation of the magnetic part of the resistivity below 300 K is consistent with that expected for single impurity Kondo behaviour. CeTiGe₃ is thus an uncommon example of a ferromagnetic dense Kondo lattice.     

Electric and magnetic signatures of martensitic and intermartensitic transformations in Ni-Mn-Ga crystal

Results are reported on the temperature dependence of resistivity (200≤T≤360 K) and low-field magnetization (5≤T≤350 K) for the off-stoichiometric Ni₄₉Mn₂₉Ga₂₂ single crystal. Measurements are made for both heating and cooling cycles. The resistivity data show two first-order (hysteretic) transformations centered at about 340 and 250 K. The magnetization data show the same two transformations as the resistivity data as well as a third centered at 285 K. The results are consistent with a martensite/austenite transformation near 340 K and two intermartensitic transformations centered at 285 and 250 K (three martensite phases).     

Properties of exchange interaction in Yb3Fe5O12 under extreme conditions

In Yb₃Fe₅O₁₂, the exchange effective field can be expressed as H_eff=−λ·M_Fe=−λχ_eff·H_e=−γ·H_e where γ is named as the exchange field parameter and H_e is the external magnetic field. Then, in this paper, by the discussions on the characteristics of the exchange field parameter γ, the properties of exchange interaction in ytterbium iron garnet (Yb₃Fe₅O₁₂) are analyzed under extreme conditions (high magnetic fields and low temperatures). Our theory suggests that the exchange field parameter γ is the function of the temperatures under different external magnetic fields, and γ=a+b·T+c·T², where the coefficients a, b, c are associated with the external magnetic fields and the magnetized directions. Thus, the temperature-dependence, field-dependence and anisotropic characteristics of the exchange interaction in Yb₃Fe₅O₁₂ are revealed. Also, excellent fits to the available experiments are obtained.     

Magnetotransport properties of (La0.7Pb0.3MnO3)1−xAgx composites

Magnetic and transport properties of (La_0.7Pb_0.3MnO₃)_1−xAg_x composites are explored in this study. Ferromagnetism is gradually attenuated due to the magnetic dilution with increase of Ag content percentage. Clearly irreversible behavior in the zero-field cooling and field cooling curves at a low field caused by the competition between the magnetization and magnetic domain orientation processes has been observed as x increases. Saturation magnetization decreases as x increases, while ferromagnetic transition temperature remains around 346 K for all composites. The resistivity decreases significantly for (La_0.7Pb_0.3MnO₃)_1−xAg_x composites. It is suggested that introduction of Ag into the niche of grain boundaries forms artificial conducting network and improves the carriers to transport. However, enhancement of magnetoresistance has been observed for the system.     

Magnetic and magnetocaloric results of magnetic field-induced transitions in La1−xCexMn2Si2 (x=0.35 and 0.45) compounds

The La_1−xCe_xMn₂Si₂ compounds (x=0.35 and 0.45) exhibit an antiferromagnetic-ferromagnetic transition caused by the changes in distance between Mn atoms due to temperature changes. A field-induced transition from antiferromagnetic state to ferromagnetic state at a critical field, which decreases with increase in temperature, can also be induced by applying a magnetic field. In this paper our aim is to study the magnetization and magnetocaloric effect, close to transition temperatures. Our subsidiary aim is to examine the temperature dependence of critical field and ferromagnetic fraction of compounds. The variation of magnetocaloric effect with temperature is correlated with the ferromagnetic-antiferromagnetic phase coexistence. Our final aim is to examine the harmony between magnetocaloric effect values calculated both by the Maxwell theory and by the Landau theory.     

Influence of temperature on critical fields in ZnCr2Se4

The complex ac dynamic magnetic susceptibility was used to study the influence of temperature on critical fields in polycrystalline ZnCr₂Se₄ spinel. An antiferromagnetic order with a Néel temperature T_N=20.7 K and a strong ferromagnetic exchange evidenced by a positive Curie-Weiss temperature θ_CW=55.1 K were established. An increasing static magnetic field shifts T_N to lower temperatures while a susceptibility peak at T_m in the paramagnetic region—to higher temperatures. The non-zero and negative values both of the second and third harmonics of susceptibility suggest only a parallel spin coupling in ferromagnetic clusters in the range between the Néel and Curie-Weiss temperatures. Below T_N the magnetic field dependence of susceptibility, χ_ac(H), shows two peaks at critical fields H_c1 and H_c2. The values of H_c1 decrease slightly with temperature while the values of H_c2 drop rapidly with temperature. The strong changes of H_c2 temperature induced are mainly responsible for a spin frustration of the re-entrant type in the spinel under study.     

Preparation and magnetoresistance of thin CrO2 films

Highly textured chromium dioxide (CrO₂) films have been deposited on Al₂O₃ single-crystal substrates by atmospheric pressure chemical vapor deposition method (CVD). X-ray diffraction patterns show that the CrO₂ films are (1 0 0)-oriented on Al₂O₃ (0 0 1) substrates, and are (1 0 1)-oriented on Al₂O₃ (0 1 2) substrates. Scanning electron microscopy images indicate that the (1 0 0)-oriented CrO₂ films grown on Al₂O₃ (0 0 1) substrates have smoother surface and better qualities than that grown on Al₂O₃ (0 1 2) substrate. At room temperature, the magnetoresistance of the (1 0 0)- and (1 0 1)-oriented CrO₂ films are nearly same, and both show a linear dependence on applied magnetic field. While at 80 K, the (1 0 1)-oriented CrO₂ films show a much larger magnetoresistance compared with the (1 0 0)-oriented CrO₂ films. The reasons are briefly discussed.     

Electronic quasiparticle structure of ferromagnetic bcc iron

We investigate the influence of electron correlations on the temperature-dependence of the electronic structure of ferromagnetic bcc iron by use of a manybody evaluation of a generalized model of magnetism. The single-particle part of the model-Hamiltonian is taken from an LDA band structure calculation. The manybody interactions are described by only two parameters, an intraband Coulomb interactionU and an interband exchangeJ. WithU=1.8 eV andJ=0.2 eV the self-consistent model solution yields aT=0 moment of about 2.04 µ_B and a Curie-temperature of 1044K. Details of the magnetic behaviour of Fe can be traced back to a striking temperature variation of the quasiparticle density of states. A novel explanation for the experimentally-observed non-collapsing exchange splitting is demonstrated in terms of the temperature-dependent spectral density for wave-vectors near the -point. Typical differences in the magnetic behaviour of Fe and Ni are worked out.     

Ab initio calculation and analysis of the properties of digital magnetic heterostructures and diluted magnetic semiconductors of IV and III-V groups

We present the first-principles calculations of digital magnetic heterostructures Si/M, Ge/M. GaAs/M, GaSb/M, GaN/M and GaN/M (50%) with M=Cr, Mn, Fe, and Co. The interaction between magnetic dopants results in a wide spin-polarized two-dimensional band inside the gap. It is found that beginning occupation of the minority-spin band greatly increases the energy of the ferromagnetic (FM) state and leads, as a rule, to the antiferromagnetic (AFM) spin ordering. This mechanism causes transition to the AFM state, when interaction between magnetic atoms is too strong, and defines the optimum of Curie temperature as a function of transition element concentration in magnetic layers.